Kanban and JIT was disruptive innovation, perhaps, when introduced by Toyota at their plant machine shop in 50s. The fact that it fits well with Agile software development made it popular today with IT companies.
While Trello is a good simple product, there is nothing disruptive about it. There were many others before and after. There was AgileZen for software developers. Then there is smartQ that was designed to use Kanban for anything, not just software development. And that’s all before Trello. What Trello did is, basically, developed a B2C product for the masses instead of B2B ones mentioned above. Simple, free (freemium) and well built. Trello did not compete with JIRA at any point, they work in different markets. FrogBugsz — their main product — did and does. Eventually, both Fogbugz and JIRA introduced Kanban views on top of their main functionality. After all, it’s just another way to present the workflow. Many PM tools are adding it as an extra feature (Asana, Wrike, etc).
JIRA did not purchase Trello because it would eventually kill JIRA. There could be multiple reasons. If they want to create a “lighter” JIRA — Trello is a great start. They bought the technology, the team, the brand and the user base. They might also bought it to just kill it (while still taking advantage of the technology and the team). After all, Rally Software did buy AgileZen right away, but did not do much with it and the original founder/developer left the company shortly after acquisition. It just did not fit into Rally technology stack.
Time will show what will happen with Trello, but, to build a next product like that and sell it for 425 mill, it also helps to be Joel Spolsky. :-)